The wrong diabetes: how C peptide testing might help

The Association of Pre-Transplant C-Peptide Level with the Development of Post-Transplant Diabetes: A Cohort Study

Background

Post-transplant diabetes mellitus (PTDM) is an important complication after kidney transplantation that results in reduced patient and allograft survival. Although there are established risk factors for PTDM, whether pretransplant C-peptide levels associate with PTDM is unknown. Therefore, in this study, we aimed to examine the association of pretransplant C-peptide levels with PTDM.

Methods

This was a cohort study of nondiabetic adult patients who underwent kidney transplant in Nova Scotia, Canada, between January 1, 2016, and March 31, 2021, with fasting C-peptide levels measured before transplant. Multivariable logistic regression was used to determine the association of pretransplant C-peptide (dichotomized around the median) with PTDM at 1 year post transplant. Given the known association between pretransplant obesity and PTDM, we repeated our primary analysis in a cohort restricted to a BMI of 20-35 kg/m².

Results

The median C-peptide value was 3251 (Q1 2480, Q3 4724); pretransplant C-peptide level was dichotomized at 3000 pmol/L. PTDM occurred in 25 (19%) individuals. Thirty percent of patients in the high and only 2% of patients in the low C-peptide groups developed PTDM (P<0.001). A C-peptide level ≥3000 pmol/L was strongly associated with PTDM in multivariable analysis (OR=18.9, 95% CI, 2.06 to 174.2). In a restricted cohort with a BMI of 20-35 kg/m², an elevated pretransplant C-peptide remained independently associated with the risk of PTDM (OR=15.7, 95% CI, 1.64 to 150.3). C-peptide was the only factor independently associated with PTDM in this restricted BMI cohort.

Conclusions

A pretransplant C-peptide level ≥3000 pmol/L was associated with a nearly 20-fold increased odds of PTDM at 1 year post kidney transplantation. Identifying patients with high pretransplant C-peptide levels may therefore help identify those at risk for PTDM who may benefit from focused preventative and therapeutic interventions and support.

Impact of routine clinic measurement of serum C-peptide in people with a clinician-diagnosis of type 1 diabetes

AIMS/HYPOTHESIS

The aim of this study was to determine the impact of the routine use of serum C-peptide in an out-patient clinic setting on individuals with a clinician-diagnosis of type 1 diabetes.

METHODS

In this single-centre study, individuals with type 1 diabetes of at least 3 years duration were offered random serum C-peptide testing at routine clinic review. A C-peptide ≥200 pmol/L prompted further evaluation of the individual using a diagnostic algorithm that included measurement of islet cell antibodies and genetic testing. Where appropriate, a trial of anti-diabetic co-therapies was considered.

RESULTS

Serum C-peptide testing was performed in 859 individuals (90% of the eligible cohort), of whom 114 (13.2%) had C-peptide ≥200 pmol/L. The cause of diabetes was reclassified in 58 individuals (6.8% of the tested cohort). The majority of reclassifications were to type 2 diabetes (44 individuals; 5.1%), with a smaller proportion of monogenic diabetes (14 individuals; 1.6%). Overall, 13 individuals (1.5%) successfully discontinued insulin, while a further 16 individuals (1.9%) had improved glycaemic control following the addition of co-therapies. The estimated total cost of the testing programme was £23,262 (~€26,053), that is, £27 (~€30) per individual tested. In current terms, the cost of prior insulin therapy in the individuals with monogenic diabetes who successfully stopped insulin was approximately £57,000 (~€64,000).

CONCLUSIONS/INTERPRETATION

Serum C-peptide testing can easily be incorporated into an out-patient clinic setting and could be a cost-effective intervention. C-peptide testing should be strongly considered in individuals with a clinician-diagnosis of type 1 diabetes of at least 3 years duration.